Indiana University-Purdue University Indianapolis (IUPUI) / Huntington’s disease (HD) is an autosomal dominantly inherited, fatal neurodegenerative disorder for which there is no cure. HD is clinically characterized by progressively worsening motor, cognitive, and psychiatric disturbances. Currently available therapeutics for HD only treat symptoms, but do not address underlying disease pathology. HD pathogenesis is linked to a mutation in the huntingtin gene, which encodes a protein called huntingtin (Htt) that is normally involved in a variety of cellular processes. In healthy individuals, the N-terminus of huntingtin possesses a polyglutamine stretch containing less than 35 glutamines, however, the mutated huntingtin protein (mHtt) has an elongated polyglutamine tract that correlates with the development of HD. The mechanism of deleterious action by mHtt is unknown, but a major hypothesis postulates that mHtt may cause mitochondrial dysfunction. However, the data regarding involvement of mitochondrial impairment in HD pathology are contradictory. Some investigators previously reported, for example, that mHtt suppresses mitochondrial respiratory activity and decreases mitochondrial Ca2+ uptake capacity. However, other investigators found increased respiratory activity and augmented mitochondrial Ca2+ uptake capacity. We used transgenic mouse models of HD to investigate the effect of full-length and fragments of mHtt on oxidative metabolism and Ca2+ handling using a combination of isolated mitochondria, primary neurons, and whole-animal metabolic measurements. We evaluated the effect of full-length mHtt on isolated mitochondria and primary neurons from YAC128 mice. We found no alteration in respiratory activity or Ca2+ uptake capacity, indicative of mitochondrial damage, between mitochondria or neurons from YAC128 mice compared to wild-type (WT) mice. Furthermore, we measured whole animal oxidative metabolism and physical activity level and found that YAC128 mice do not display any decline in metabolic and physical activity. Although full-length mHtt expressing YAC128 mice may be a more faithful genetic recapitulation of HD, data suggests mHtt fragments may be more toxic. To assess the effect of mHtt fragments, we used isolated brain mitochondria and primary striatal neurons from the R6/2 mouse model and found no significant impairment in respiration or Ca2+ handling. Thus, our data strongly support the hypothesis that mHtt does not alter mitochondrial functions assessed either with isolated mitochondria, primary neurons, or whole animals.

Une dysfonction olfactive et une altération de la neurogenèse sont observées dans plusieurs maladies neurodégénératives, y compris la maladie de Huntington (MH). Ce déficit est un symptôme précoce de la MH et est en corrélation avec le déclin de la performance cognitive globale, la dépression et la dégénérescence des régions olfactives dans le cerveau. La dysfonction olfactive dans les maladies neurodégénératives est souvent accompagnée par des anomalies structurelles de l’épithélium olfactif, du bulbe olfactif (BO) et du cortex olfactif chez l’humain. En dépit de preuves claires démontrant la dysfonction olfactive chez les patients de la MH l'information disponible est limitée dans des modèles murins et les mécanismes sous-jacents ne sont pas connus. Une diminution du volume et du compte neuronale dans le PC est observée dans les souris YAC128 vs le type sauvage (WT) âgée (12 mois). Nous avons également examiné les comportements lors d'exposition à des odeurs sociales et non sociales chez des souris en utilisant le test habituation. Une habituation aux odeurs tend à être observé dans les souris YAC128 de 1 mois vs WT se traduisant par une tendance de l’augmentation de la durée d’exploration des YAC128 vs WT lors de leurs 2e et 3e expositions à une odeur. Dans les couches glomérulaire et plexiforme externe, l’intensité réciproque du marquage de TH et de GFAP, marqueur de cellules gliales présente une tendance pour une augmentation dans les YAC128 comparativement au WT. Une tendance pour une diminution de Iba-1, marqueur de neuroinflammation, a aussi été observé dans la couche granulaire du BO de YAC128 âgée vs WT. Malgré une diminution de l’expression en ARNm de caspase-3 et -8, une augmentation de l'expression protéique de la proforme de caspase-8 et des formes actives de la caspase-8, -6 et -9 a été observés au stade présymptomatique dans des BO de YAC128 vs WT. De façon similaire aux YAC128, une atrophie du BO à 6 mois est remarqué dans le modèle de rat BACHD (lignées TG5 et TG9). Globalement, un niveau d’expression de la protéine huntington mutante (httm) est plus élevé dans les TG5. L’expression protéique dans les BO de la proforme des caspase-3, -6 et -8 sont supérieurs dans les TG9 lorsque comparés au TG5 et au WT. L’identification de marqueurs précoces pour la MH contribuera aux approches thérapeutiques et permettra de clarifier l'utilité des tests de la fonction olfactive dans les individus à risques de la MH.

Indiana University-Purdue University Indianapolis (IUPUI) / Huntington’s Disease (HD) is an inherited, autosomal dominant, neurodegenerative disorder. There is no cure for HD and the existing therapies only alleviate HD symptoms without eliminating the cause of this neuropathology. HD is linked to a mutation in the huntingtin gene, which results in an elongation of the poly-glutamine stretch in the huntingtin protein (Htt). A major hypothesis is that mutant Htt (mHtt) leads to aberrant Ca2+ homeostasis in affected neurons. This may be caused by increased Ca2+ influx into the cell via the N-methyl-Daspartate (NMDA)-subtype of glutamate receptors. The contribution of two major Ca2+ removal mechanisms, mitochondria and plasmalemmal Na+/Ca2+ exchangers (NCX), in neuronal injury in HD remains unclear. We investigated Ca2+ uptake capacity in isolated synaptic (neuronal) and nonsynaptic mitochondria from the YAC128 mouse model of HD. We found that both Htt and mHtt bind to brain mitochondria and the amount of mitochondriabound mHtt correlates with increased mitochondrial Ca2+ uptake capacity. Mitochondrial Ca2+ accumulation was not impaired in striatal neurons from YAC128 mice. We also found that expression of the NCX1 isoform is increased with age in striatum from YAC128 mice compared to striatum from wild-type mice. Interestingly, mHtt and Htt bind to the NCX3 isoform but not to NCX1. NCX3 expression remains unchanged. To further investigate Ca2+ homeostasis modulation, we examined the role of collapsin response mediator protein 2 (CRMP2) in wild-type neurons. CRMP2 is viewed as an axon guidance protein, but has been found to be involved in Ca2+ signaling. We found that CRMP2 interacts with NMDA receptors (NMDAR) and disrupting this interaction decreases NMDAR activity. CRMP2 also interacts with and regulates NCX3, resulting in NCX3 internalization and decreased activity. Augmented mitochondrial Ca2+ uptake capacity and an increased expression of NCX1 in the presence of mHtt suggest a compensatory reaction in response to increased Ca2+ influx into the cell. The role of NCX warrants further investigation in HD. The novel interactions of CRMP2 with NMDAR and NCX3 provide additional insight into the complexity of Ca2+ homeostasis regulation in neurons and may also be important in HD neuropathology.